In order to perform beam forming, a plurality of antenna elements are required. The higher frequencies that are used for communication, the more antenna elements, coherently combined, are needed to get a sufficiently large antenna area. One of the problems of having a large number of antenna elements is the handling of all the received data before the necessary combination has been done.
The concept of beam forming is illustrated in a schematic manner in FIG. 1, which shows a network node 101 performing beam forming using an antenna arrangement comprising 8×8 antenna elements. The network node provides a set of wireless devices 101-103 with radio coverage by means of a set of beams 105-107. As illustrated in FIG. 1, there may be wireless devices 108 within an area coverable by the network node, which are not provided with coverage for various reasons.
A transceiver, such as a radio base station, RBS, typically comprises radio equipment, RE and a radio equipment controller, REC, as illustrated in FIG. 2. The antenna elements are comprised in the RE, and received data is conveyed to the REC via an interface, typically having a limited capacity. One way of reducing the large amount of received data is that the radio equipment, RE, of a transceiver performs beam forming and sends only requested data comprised in virtual beams to the UL physical layer processing.
Another alternative is to allocate extra bandwidth, or capacity, to the interface for sending more beams, or less narrow beams, and then do final beam forming in the REC.
If the radio, RE, combines the antenna signals, i.e. performs the beam forming, rays falling outside the beams will not be known to the physical layer processing, and will thus not be asked for. This will reduce how aggressive the beam forming can be, i.e. how narrow beams or how few beams that can be used. Further, allocating extra bandwidth to the interface for extra beams is costly.
Below, a beam forming radio base station, RBS, for which the solution described herein is applicable, will be described.
The RBS comprises two main parts—the Radio Equipment Controller, REC, and the Radio Equipment, RE, as illustrated in FIG. 2. Although these terms are derived from the Common Public Radio Interface, CPRI, specification, the functional allocation of the RBS described here is somewhat different as compared to what is described in the CPRI specification. Some differences are that:                The REC does not send antenna streams to the RE, but rather MIMO streams, or virtual antenna streams. In CPRI, the REC can directly address the RE antennas, but in this beam forming RBS, the RE includes the functionality of mapping a MIMO stream to a set of physical antennas in order to generate a wanted beam form. Sometimes, this is called virtual antenna ports, and may be regarded as that the RE presents a set of virtual antenna patterns of which the REC can chose from. The REC thus sends the RE a data stream and information on what virtual antenna to output it on. This can be different for different simultaneous UEs, and different for UL and DL.        To have an efficient simultaneous multi user beam forming, the FFT and IFFT functions are moved to the RE. In addition, the beam forming functionality is added to the RE.        The REC-RE interface is typically no longer a streaming interface, but packet oriented, sending the (frequency domain) samples to the RE symbol by symbol. This allows for quick and flexible allocation of resources on the interface to different users. This is not a necessity, but at least the beam forming information is packet based.        
The REC still maintain the knowledge about the mobile users, such as data channel, beam direction, etc, and the RE acts solely on commands from the REC.
If considering a system with 400 MHz air interface band width, support for 4 MIMO streams and utilizing 64 antennas for beam forming:
A CPRI realization of this system exposing all 64 antennas for the REC would require approximately 64 CPRI interfaces of 10 Gbps, since a CPRI interface carries about 400 MHz. Further, an interface using virtual antenna ports would require 4 MIMO streams of 400 MHz, and would thus require about 4 interfaces of 10 Gbps, since one 10 Gbps interface still carries data for about 400 MHz.